Synthetic polyester fibers have been known and used commercially for several decades, having been first suggested by W. H. Carothers, U.S. Pat. No. 2,071,251, and then by Whinfield and Dickson, U.S. Pat. No. 2,465,319. Most of the polyester polymer that has been manufactured and used commercially has been poly(ethylene terephthalate), sometimes referred to as 2G-T. This polymer is often referred to as homopolymer. Commercial homopolymer is notoriously difficult to dye. Such homopolymer is mostly dyed with disperse dyestuffs at high temperatures under elevated pressures, which is a relatively expensive and inconvenient process (in contrast to processes for dyeing several other commercial fibers at atmospheric pressure, e.g. at the boil), and so there have been several suggestions for improving the dyeability of polyester yarns. For instance, Griffing and Remington, U.S. Pat. No. 3,018,272, suggested the use of cationic-dyeable polyesters. Such polyesters, consisting essentially of poly [ethylene terephthalate/5-(sodium sulfo) isophthalate] containing about 2 mol % isophthalate groups in the polymer chain (2G-T/SSI), have been used commercially as a basis for polyester yarns for some 20 years.
Although such polyester fibers have been very useful, it has long been desirable to provide alternative fibers, having the desirable characteristics of commercial polyester fibers accompanied by excellent dyeing properties.
Watson, in U.S. Pat. No. 3,385,831, suggested textile fibers of copolymers of polyethylene terephthalate/hexahydroterephthalate. These fibers showed a surprising combination of enhanced dyeability and good overall physical properties, including low shrinkage values. These copolymer fibers are rather unique, considering the unusually large molar amounts of comonomer (i.e. the hexahydroterephthalate units, HT) in comparison with other comonomers in polymers with ethylene terephthalate (2G-T). Despite the advantages on paper, however, Watson's fibers were not produced in commercial quantities. Some reasons are believed to be the relatively poor aesthetics and relatively high sensitivity to elevated temperatures of Watson's fibers, especially when processed in the form of fabrics. As indicated, several properties do get less desirable as the proportion of comonomer is increased, although the dyeability is correspondingly improved. The improved dyeability from higher proportions of HT comonomers would have been very desirable, if certain problems could have been solved, with regard to fabric aesthetics, especially.
An object of the present invention is to improve the properties of Watson's type of fibers of copolymers containing ethylene terephthalate (2G-T) and ethylene hexahydroterephthalate (2G-HT) units, especially in the textile fabrics in which form they are ultimately used.